To improve bottom coverage of high aspect ratio vias, channels and other openings in a wafer or other substrate during a deposition process, the deposition material may be ionized in a plasma prior to being deposited onto the substrate. The ionized deposition material may be redirected by electric fields to ensure more material reaches the bottom areas. It has been found that it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density, in the range of 10.sup.11 -10.sup.13 ions/cm.sup.3. Such a plasma is also useful for other semiconductor processes such as etching a wafer.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through an antenna in the form of a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
As described in copending application Ser. No. 08/680,335, entitled "Coils for Generating a Plasma and for Sputtering," filed Jul. 10, 1996 and assigned to the assignee of the present application, it has been recognized that the coil itself may provide a source of sputtered material to supplement the deposition material sputtered from the primary target of the chamber. Application of an RF signal to a coil positioned in the chamber can cause the coil to develop a negative bias which will attract positive ions which can impact the coil causing material to be sputtered from the coil.
Because relatively large currents are passed through the coil to energize the plasma, the coil often undergoes significant resistive heating. In addition, ions impacting the coil can further heat the coil if the coil is used as a sputtering source. As a result, an internal coil can reach relatively high temperatures which can have an adverse effect on the wafer, the wafer deposition process or even the coil itself. Moreover, the coil will cool once the deposition is completed and the current to the coil is removed. Each heating and subsequent cooling of the coil causes the coil to expand and then contract. This thermal cycling of the coil can cause target material deposited onto the coil to generate particulate matter which can fall onto and contaminate the wafer.
To reduce coil heating, it has been proposed in some applications to form the coil from hollow tubing through which a coolant such as water is passed. However, because the source of coolant is most conveniently located outside the chamber, the vacuum chamber in which the coil is located will typically require a feedthrough to permit coolant to pass through the chamber wall, through the coil and back to the exterior of the chamber. In addition, because the RF source may be located exterior to the chamber as well, a feedthrough for the RF power to the coil may also be needed in the chamber wall. However, the chamber wall is usually maintained at ground potential for safety and other reasons. Hence, the RF feedthrough should be capable of electrically insulating the coil from the chamber wall. Still further, the coolant and RF feedthroughs should be capable of maintaining a large pressure differential between the exterior of the chamber which is typically at ambient pressure, and the interior of the chamber which may be at 1 milliTorr or lower pressure. As a consequence, known RF and fluid feedthroughs tend to be relatively complex and difficult to install.
For example, one known feedthrough comprises a conduit having two ends, one external and one internal. Once the feedthrough is installed in the chamber wall, the coil may be welded or otherwise joined to the internal end and sources of RF energy and coolant are coupled to the external end. However, it has been recognized by the present applicant that the internal joint between the coil and the feedthrough is a potential leakage point which could significantly disrupt the semiconductor processes performed in the chamber and potentially damage the chamber itself.